Gasoline, the fuel for many automotive vehicles, is a volatile liquid subject to potentially rapid evaporation, in response to diurnal variations in the ambient temperature. Thus, the fuel contained in automobile gas tanks presents a major source of potential emission of hydrocarbons into the atmosphere. Such emissions from vehicles are termed ‘evaporative emissions’ and those vapors can be emitted vapors even when the engine is not running
In response to this problem, industry has incorporated evaporative emission control systems (EVAP) into automobiles, to prevent fuel vapor from being discharged into the atmosphere. EVAP systems include a canister (the carbon canister containing adsorbent carbon) that traps fuel vapor. Periodically, a purge cycle feeds the captured vapor to the intake manifold for combustion, thus reducing evaporative emissions.
Hybrid electric vehicles, including plug-in hybrid electric vehicles (HEV's or PHEV's), pose a particular problem for effectively controlling evaporative emissions. Although hybrid vehicles have been proposed and introduced in a number of forms, these designs all provide a combustion engine as backup to an electric motor. Primary power is provided by the electric motor, and careful attention to charging cycles can produce an operating profile in which the engine is only run for short periods. Systems in which the engine is only operated once or twice every few weeks are not uncommon. Purging the carbon canister can only occur when the engine is running, of course, and if the canister is not purged, the carbon pellets can become saturated, after which hydrocarbons will escape to the atmosphere, causing pollution.
EVAP systems are generally sealed to prevent the escape of any hydrocarbons. These systems require periodic leak detection tests to identify potential problems. Different system suppliers have adopted different testing methods, which can be generally classified as either vacuum-based or pressure-based techniques.
Vacuum-based techniques rely on evacuating the EVAP system and then monitoring to determine whether the system can hold the vacuum without bleed-up. This technique is known to produce false failures, unfortunately. More particularly, when the system is evacuated, air and vapor are removed, but once the system is resealed, the partial pressures of the fuel and vapor dome tend to equalize, resulting in a pressure rise. Usually, it is difficult to discern whether a bleed-up exists because of a leak or partial pressure equalization. Vacuum-based systems however, do not emit hydrocarbons into the atmosphere.
Pressure-based techniques are more reliable and less prone to false failures. In this procedure, when the system is pressurized, pressure conditions within the system mostly remain constant even when the pressure source is removed. Given the fact that the system is pressurized, however, this technique is susceptible to hydrocarbon release.
No EVAP leak detection system or method is currently available in the art that substantially minimizes release of hydrocarbons into the atmosphere, while also minimizing false failures during leak tests.